Aqueous inks have become the major ink for ink-jet recording use other than an industrial use because the danger of fire and toxicity such as mutagenicity, which are peculiar to oil-based inks, can be reduced. In aqueous inks, dyes have been used as coloring agents because stability is high, nozzle clogging is reduced, color-developing properties is favorable, and printing with high image quality is possible. However, dyes have the problems of poor water resistance and light fastness. In particular, with an increase of the case where a printed matter is used outdoors for industrial use or indoors for long-term ornamental purposes, excellent water resistance and light fastness are required for pictures formed of an ink-jet recording ink.
In order to solve this problem, the conversion from dyes to pigments has been actively performed. However, in pigment inks, nozzle clogging is likely to occur due to the aggregation/sedimentation of pigments although excellent water resistance and light fastness can be expected. Therefore, the method has been investigated, which disperses pigments in an aqueous medium by using polymer-based dispersing agents so as to obtain favorable dispersion stability and favorable discharging properties during ink-jet recording.
In particular, in recent years, investigations have been performed, which use inks having a color tone such as red or violet in addition to the 4 colored inks of black, cyan, magenta, and yellow, thereby increasing the color zone of a printed image and obtaining a favorable color reproduction. Therefore, regarding a color tone other than the conventional basic 4 colors, which is called a special color, an attempt is performed, which uses pigments so as to improve the water resistance and light fastness of pictures formed of an ink-jet recording ink.
Violet is one of the color tones used as such special colors. Also, C.I. pigment violet 23 is a violet pigment which is excellent in color-developing properties and light fastness, and there are the reports of ink-jet recording inks which uses C.I. pigment violet 23 (see patent references 1 and 2).
In the case where the aforementioned pigment is used in an ink-jet ink, importance is attached to high stability which is enough to maintain an average particle diameter and the viscosity of pigment particles dispersed in the liquid of an ink for a long time particularly even under high-temperature conditions of 50° C. or more. In particular, in the case where ink-jet recording is performed by using an ink-jet printer of a thermal jet recording system, the aforementioned high-temperature preservation stability is essential. However, in C.I. pigment violet 23, an anionic group-containing resin, which is favorably used as a dispersing resin, for example a styrene-acrylic acid-based resin, is easily eliminated from the pigment surface in comparison with other pigments. In an ink-jet recording ink produced by the methods described in the aforementioned patent references 1 and 2, sufficient stability cannot always be obtained.
In the methods described in the patent reference 1, C.I. pigment violet 23 is dispersed by using a styrene-(meth)acrylic acid-based water-soluble resin as a dispersing agent. Dispersion is performed by using a sand mill, but there is no kneading step followed by the dispersion. Therefore, resin-coating of the pigment surface of C.I. pigment violet 23, from which resins are easily eliminated primarily, is not rigidly performed, and the resins are easily eliminated. Thus, sufficient dispersion-stability cannot be ensured.
Also, the patent reference 2 discloses a production method of an aqueous pigment dispersion liquid which uses a kneading step, and an apparatus and a dispersion liquid which are suitable for this production method. In addition, C.I. pigment violet 23 is also exemplified as a usable pigment, and a styrene-acrylic acid-based resin is used as a resin. However, no example is disclosed on the production of an aqueous pigment dispersion liquid using C.I. pigment violet 23. Even though a styrene-acrylic acid-based resin is extremely easily eliminated from the pigment surface of C.I. pigment violet 23 in comparison with other pigments, the specific investigation, in which this particularity is considered, is not performed. Accordingly, regarding a production method of an aqueous pigment dispersion liquid, particularly a kneading step, the selection of raw materials, the optimization of composition, and the investigation of step conditions are not performed. Therefore, the disclosed production method is not sufficient for sufficiently fulfilling long-term preservation stability at a high temperature.
In this way, even though C.I. pigment violet 23 has a favorable light fastness, in the conventional production method, a resin was not able to rigidly attach to the pigment surface of C.I. pigment violet 23, and it was impossible to form an ink-jet recording ink having favorable dispersion stability. Therefore, aggregation frequently occurs during stable long-term preservation at high temperatures, and sufficient discharging stability, preservation stability, and a luster were not able to be obtained particularly when an ink was used for printing and image formation using thermal jet recording.    [Patent Reference 1] Japanese Unexamined Patent Application, First Publication No. 2004-217765    [Patent Reference 2] Japanese Unexamined Patent Application, First Publication No. 2004-143316